Myopia is a visual defect in which distant objects appear blurred because their images are focused in front of the retina rather than on it. The term myopia encompasses all forms of myopia including, but not limited to, axial myopia, refractive myopia, myopic astigmatism and simple myopia. Myopia is also known as nearsightedness or short sightedness. Some myopia is associated with astigmatism which is often the result of an unequal curvature of the cornea of the eye which prevents light rays from focusing clearly at one or more points in the retina resulting in blurred vision. Myopia is a common visual disorder. Myopic progression is the deterioration of the myopic condition, so that a person becomes more short sighted. Myopia, and myopic progression, are associated with a greater risk of myopic retinal degeneration, glaucoma, and retinal detachment. Further, in developed countries myopia is currently the fifth most common cause of registered blindness.
Increased risk factors for myopia include myopic parents, the amount of near work, early visual deprivation and ethnicity; with Asians having significantly higher rates of myopia than Caucasians. Optical correction, such as eyeglasses, contact lenses, and refractive surgery for myopia is a major health care expense.
The development of myopia is often characterised as being of either axial or refractive origin. In axial myopia, the eye grows too long so the distance between the front surface of the cornea and the retina is too long compared to the optical refractive power of the eye. This elongation typically occurs in the vitreous chamber depth which is the distance between the back surface of the crystalline lens and the retina. In the less common refractive myopia the optical power of the eye, primarily the refractive power of the cornea and crystalline lens, are too strong compared to the axial length of the eye.
Astigmatism generally results from the cornea growing asymmetrically to produce corneal astigmatism, although it can also arise through the optical characteristics of the crystalline lens.
The control of eye growth may therefore occur through a range of mechanisms including axial length of the eye, corneal refractive power or the refractive power of the crystalline lens inside the eye. Failure of the mechanisms which are thought to regulate natural axial growth of the eye and the refractive power of the optical components of the eye may therefore result in the common refractive errors, such as, simple myopia, simple hyperopia, myopic astigmatism, hyperopic astigmatism and mixed astigmatism.
Previous attempts at myopia control have included spectacles, pharmacological methods, and contact lenses. The spectacle based therapies have included bifocals, near Rx (near prescription lenses), and progressive lenses.
The pharmacological approaches for myopia control have included atropine and pirenzepine. Atropine is a drug which paralyses accommodation and has been shown to slow myopia progression, however it is not a practical treatment. Pirenzepine, a selective M1-muscarinic antagonist has been shown to reduce myopic progression over a one-year period, however, subsequent results suggest that the effects of pirenzepine are limited.
Spectacle based approaches for myopia control also have disadvantages as some people prefer to wear contact lenses as they believe they are more attractive without spectacles, or do not want to be encumbered by spectacles or have better peripheral vision with contact lenses. Additionally, contact lenses are preferable for many active endeavors such as sports.
The contact lens based approaches for myopia control have been confined to rigid or hard lenses and orthokeratology. Orthokeratology is the use of contact lenses to temporarily reshape the cornea of the eye with the goal of achieving sharper vision.
A number of studies have been conducted into hard contact lenses and myopic progression, however while the results show some evidence for hard contact lenses slowing myopic progression, the results are inconclusive. Regardless, hard contact lenses are sometimes uncomfortable for the wearer, which for the comparatively sensitive eye, results in non-compliance.
U.S. Pat. No. 6,045,578 identifies a potential method of eye growth control based on a particular optical aberration of the eye, spherical aberration, U.S. Pat. No. 6,045,578 discloses how the presence of negative spherical aberration could promote eye growth and shows how correcting the negative spherical aberration of the eye could therefore slow or arrest eye growth. U.S. Pat. No. 6,045,578 does not address specific causes of myopia or myopia development associated with downward gaze and near work. Accordingly there is a need for a method and device which addresses these specific causes of myopia.
In this specification, the terms “comprises”, “comprising” or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.